Production of methanol via catalytic coal gasification

ABSTRACT

Methanol is produced by gasifying a carbonaceous feed material with steam in the presence of a carbon-alkali metal catalyst and added hydrogen and carbon monoxide at a temperature between about 1000° F. and about 1500° F. and at a pressure in excess of about 100 psia to produce a raw product gas comprising methane, steam, carbon dioxide, carbon monoxide, hydrogen and hydrogen sulfide; withdrawing the raw product gas from the gasifier and treating it for the removal of steam, particulates, hydrogen sulfide and carbon dioxide to produce a treated gas containing primarily carbon monoxide, hydrogen and methane; separating the treated gas into a methane-rich gas stream and a gas stream containing primarily carbon monoxide and hydrogen; passing the gas stream containing primarily carbon monoxide and hydrogen to a methanol synthesis reactor where the carbon monoxide is reacted with the hydrogen in the presence of a methanol synthesis catalyst to form methanol; recovering methanol product from the effluent exiting the methanol synthesis reactor thereby leaving a gas comprised of carbon monoxide, hydrogen, methane and carbon dioxide; and passing a portion of this gas to a steam reforming furnace wherein at least a portion of the methane is reacted with steam to produce hydrogen and carbon monoxide which is then passed from the steam reforming furnace into the gasifier. Preferably, at least a portion of the methane-rich gas produced in the separation step is used as fuel for the steam reforming furnace.

BACKGROUND OF THE INVENTION

This invention relates to the gasification of coal and similarcarbonaceous materials and is particularly concerned with producingmethanol by integrating a catalytic gasification process carried out inthe presence of a carbon-alkali metal catalyst with a methanol synthesisprocess.

Existing and proposed processes for the manufacture of synthetic gaseousfuels from coal or similar carbonaceous materials normally require thereaction of carbon with steam, alone or in combination with oxygen, attemperatures between about 1200° F. and about 2500° F. to produce a gaswhich may contain some methane but consists primarily of hydrogen andcarbon monoxide. This gas can be used directly as a synthesis gas or afuel gas with little added processing or can be reacted with additionalsteam to increase the hydrogen-to-carbon monoxide ratio and then fed toa catalytic methanation unit for reaction with carbon monoxide andhydrogen to produce methane. It has been shown that processes of thistype can be improved by carrying out the initial gasification step inthe presence of a catalyst containing an alkali metal constituent. Thealkali metal constituent accelerates the steam-carbon gasificationreaction and thus permits the generation of synthesis gas at somewhatlower temperatures than would otherwise be required. Processes of thistype are costly because of the large quantities of heat that must besupplied to sustain the highly endothermic steam-carbon reaction. Onemethod of supplying this heat is to inject oxygen directly into thegasifier and burn a portion of the carbon in the feed material beinggasified. This method is highly expensive in that it requires existenceof a plant to manufacture the oxygen. Other methods for supplying theheat have been suggested, but these, like that of injecting oxygen, areexpensive.

It has been found that difficulties associated with processes of thetype described above, can largely be avoided by carrying out thereaction of steam with carbon in the presence of a carbon-alkali metalcatalyst and substantially equilibrium quantities of added hydrogen andcarbon monoxide. Laboratory work and pilot plant tests have shown thatcatalysts produced by the reaction of carbon and alkali metal compoundssuch as potassium carbonate to form carbon-alkali metal compounds orcomplexes will, under the proper reaction conditions, equilibrate thegas phase reactions occurring during gasification to produce methane andat the same time supply substantial amounts of exothermic heat withinthe gasifier. This additional exothermic heat of reaction essentiallybalances the overall endothermicity of the reactions involving solidcarbon and thus results in a substantially thermoneutral process inwhich the injection of large amounts of oxygen or the use of otherexpensive methods of supplying heat are eliminated.

The catalytic effect of carbon-alkali metal catalysts on the gas phasereactions, as distinguished from the solid-gas reactions or thereactions of carbon with steam, hydrogen or carbon dioxide, allows thefollowing exothermic reactions to contribute substantially to thepresence of methane in the effluent gas and drastically reduces theendothermicity of the overall reaction:

    2CO+2H.sub.2 →CO.sub.2 +CH.sub.4 (exothermic)       (1)

    CO+3H.sub.2 →H.sub.2 O+CH.sub.4 (exothermic)        (2)

    CO.sub.2 +4H.sub.2 →2H.sub.2 O+CH.sub.4 (exothermic) (3)

Under the proper operating conditions, these reactions can be made totake place within the gasification zone and supply large amounts ofmethane and additional exothermic heat which would otherwise have to besupplied by the injection of oxygen or other means. Laboratory and pilotplant tests have shown that constituents of the raw product gas thusproduced are present in equilibrium concentrations at reactionconditions and consist primarily of hydrogen, carbon monoxide, carbondioxide, methane and steam.

It has been proposed in U.S. Pat. No. 4,211,669 to utilize steamgasification in the presence of a carbon-alkali metal catalyst toproduce a chemical synthesis gas by treating the raw product gaswithdrawn from the gasifier for removal of steam and acid gases,principally carbon dioxide and hydrogen sulfide; cryogenicallyseparating carbon monoxide and hydrogen in amounts equivalent to theirequilibrium concentration in the raw product gas from the methane in thetreated gas; withdrawing the carbon monoxide and hydrogen as chemicalsynthesis product gas; contacting the methane with steam in a steamreformer under conditions such that at least a portion of the methanereacts with steam to produce hydrogen and carbon monoxide; and passingthe effluent from the reformer into the gasifier. The reformer effluentwill normally contain carbon monoxide and hydrogen in amounts equivalentto the equilibrium quantities of those gases present in the raw productgas and will therefore supply the substantially equilibrium quantitiesof hydrogen and carbon monoxide required in the gasifier along with thecarbon-alkali metal catalyst and steam to produce the thermoneutralreaction that results in the formation of essentially methane and carbonmonoxide.

As evidenced by U.S. Pat. Nos. 4,094,650 and 4,118,204, respectively, ithas also been proposed to utilize steam gasification of a carbonaceousfeed material in the presence of a carbon-alkali metal catalyst toproduce both a high Btu and an intermediate Btu product gas. Theseprocesses are somewhat similar to the one described in U.S. Pat. No.4,211,669. In the process disclosed in U.S. Pat. No. 4,094,650, themethane from the cryogenic separation step is recovered as product andthe carbon monoxide and hydrogen are recycled to the gasifier to providethe required equilibrium quantities of hydrogen and carbon monoxide. Inthe process described in U.S. Pat. No. 4,118,204, the cryogenicseparation step is eliminated and a portion of the carbon monoxide,hydrogen and methane exiting the acid gas removal step is recovered asthe intermediate Btu product gas and the remainder is passed through asteam reformer to convert the methane into carbon monoxide and hydrogen.The effluent from the reformer is then passed into the gasifier tosupply the required amounts of carbon monoxide and hydrogen.

Although the above-described catalytic gasification processes result inthe substantially thermoneutral reaction of steam with carbon to form araw product gas containing equilibrium quantities of carbon monoxide,carbon dioxide, hydrogen, steam and methane by recycling carbon monoxideand hydrogen in quantities equivalent to their concentration in the rawproduct gas to the gasifier and are therefore significant improvementsover previously proposed non-catalytic and catalytic processes, theyhave one major disadvantage. None of the processes can be operated in amanner to produce liquid hydrocarbons. Since there may be a great needin the future for storable synthetic liquids that can be used to fuelvehicles, it may be highly desirable to utilize the thermoneutralprocess for gasifying carbonaceous materials described above in a mannerwhich would allow the production of liquids instead of gases.

SUMMARY OF THE INVENTION

This invention provides a process for producing methanol by thesubstantially thermoneutral reaction of steam with coal, petroleum coke,heavy oil, residua and other carbonaceous feed materials in the presenceof a carbon-alkali metal catalyst and added hydrogen and carbonmonoxide. In accordance with the invention, it has now been found thatmethanol can be produced by gasifying a carbonaceous feed material withsteam in a gasification zone at a temperature between about 1000° F. andabout 1500° F. and at a pressure in excess of about 100 psia in thepresence of a carbon-alkali metal catalyst and added hydrogen and carbonmonoxide, thereby producing an effluent gas containing methane, carbonmonoxide, steam, hydrogen, carbon dioxide and hydrogen sulfide. Theeffluent gas is withdrawn from the gasification zone and treated for theremoval of particulates, steam, carbon dioxide and hydrogen sulfide toproduce a treated gas containing primarily carbon monoxide, hydrogen andmethane. The treated gas is then passed to a separation zone where it isdivided into a gas stream containing primarily carbon monoxide andhydrogen and a methane-rich gas stream. The gas stream containingprimarily carbon monoxide and hydrogen is passed to a methanol synthesiszone wherein a portion of the carbon monoxide and hydrogen is reacted inthe presence of a methanol synthesis catalyst to form methanol. Theeffluent gas from the methanol synthesis zone is treated to recovermethanol product, thereby leaving a gas comprised primarily of carbonmonoxide, hydrogen, methane and carbon dioxide. At least a portion ofthis gas, which portion is commonly referred to as purge gas, iscontacted with steam in a steam reforming zone under conditions suchthat at least a portion of the methane reacts with steam to producecarbon monoxide and hydrogen. The effluent from the steam reforming zoneis then passed into the gasification zone to supply the required amountsof added hydrogen and carbon monoxide. Preferably a portion of themethane-rich gas produced in the separation zone is used as fuel for thereforming zone.

In a preferred embodiment of the invention, the portion of the gas leftafter the methanol is recovered from the methanol synthesis zoneeffluent that is not passed to the steam reforming zone of the processis recycled directly to the methanol synthesis zone. In some instancesit may be desirable to use a portion of the methane-rich gas produced inthe separation zone of the process as supplemental feed to the steamreforming zone or as a by-product gas of an intermediate Btu or high Btuheating value. In some cases, it may also be preferable to add a portionof the carbon dioxide removed from the gasification zone effluent to themethanol synthesis zone feed in order to adjust the carbon oxides tohydrogen ratio and/or to activate the methanol synthesis catalyst. It isnormally desirable that the steam reforming effluent contain carbonmonoxide and hydrogen in amounts equivalent to the quantities of thosegases present in the effluent gas withdrawn from the gasification zoneso that the effluent from the steam reforming zone will supply thequantities of hydrogen and carbon monoxide required in the gasificationzone along with the carbon-alkali metal catalyst and steam to producethe thermoneutral reaction that results in the formation of essentiallymethane and carbon dioxide.

The process of the invention provides a highly efficient method ofintegrating a thermoneutral gasification process with a methanolsynthesis in order to produce methanol and therefore has many advantagesover thermoneutral gasification processes in the past which could beused only to produce gaseous products.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic flow diagram of a process carried out inaccordance with the invention for the production of methanol via thecatalytic gasification of coal or similar carbonaceous solids.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process depicted in the drawing is one for the production ofmethanol by the gasification of bituminous coal, subbituminous coal,lignitic coal, coal char, coke, liquefaction bottoms, oil shale andsimilar carbonaceous solids with steam at a high temperature in thepresence of a carbon-alkali metal catalyst prepared by impregnating thefeed solids with a solution of an alkali metal compound or mixture ofsuch compounds and thereafter heating the impregnated material to atemperature sufficient to produce an interaction between the alkalimetal and the carbon present. The solid feed material that has beencrushed to a particle size of about 8 mesh or smaller on the U.S. SieveSeries Scale is passed into line 10 from a feed preparation plant orstorage facility that is not shown in the drawing. The solids introducedinto line 10 are fed into a hopper or similar vessel 11 from which theyare passed through line 12 into feed preparation zone 14. This zonecontains a screw conveyor or similar device, not shown in the drawing,that is powered by a motor 16, a series of spray nozzles or similardevices 17 for the spraying of an alkali metal-containing solutionsupplied through line 18 onto the solids as they are moved through thepreparation zone by the conveyor, and a similar set of nozzles or thelike 19 for the introduction of a hot dry gas, such as flue gas, intothe preparation zone. The hot gas, supplied through line 20, serves toheat the impregnated solids and drive off the moisture. A mixture ofwater vapor and gas is withdrawn from zone 14 through line 21 and passedto a condensor, not shown, from which water may be recovered for use asmakeup or the like. The majority of the alkali metal-containing solutionis recycled through line 49 from the alkali metal recovery portion ofthe process, which is described in more detail hereinafter. Any makeupalkali metal solution required may be introduced into line 18 via line13.

It is preferred that sufficient alkali metal-containing solution beintroduced into preparation zone 14 to provide from about 5 to about 50weight percent of an alkali metal compound or mixture of such compoundson the coal or other carbonaceous solids. From about 10 to about 30percent is generally adequate. The dried impregnated solid particlesprepared in zone 14 are withdrawn through line 24 and passed to a closedhopper or similar vessel 25 from which they are discharged through astar wheel feeder or equivalent device 26 in line 27 at an elevatedpressure sufficient to permit their entrainment into a stream of highpressure steam, recycle product gas, inert gas or other carrier gasintroduced into line 29 via line 28. The carrier gas and entrainedsolids are passed through line 29 into manifold 30 and fed from themanifold through feed lines 31 and nozzles, not shown in the drawing,into gasifier 32. In lieu of or in addition to hopper 25 and star wheelfeeder 26, the feed system may employ parallel lock hoppers, pressurizedhoppers, aerated standpipes operated in series, or other apparatus toraise the input feed solids stream to the required pressure level.

Gasifier 32 contains a fluidized bed of carbonaceous solids extendingupward within the vessel above an internal grid or similar distributiondevice not shown in the drawing. The bed is maintained in the fluidizedstate by means of steam, hydrogen and carbon monoxide introduced throughline 33, manifold 34 and peripherally spaced injection lines and nozzles35, and through bottom inlet line 36. The particular injection systemshown in the drawing is not critical and hence other methods forinjecting the steam, hydrogen and carbon monoxide may be employed. Insome instances, for example, it may be preferred to introduce the gasesthrough multiple nozzles to obtain more uniform distribution of injectedfluid and reduce the possibility of channeling and related problems.

Within the fluidized bed in gasifier 32, the carbonaceous solidsimpregnated with an alkali metal compound or mixture of such compoundsare subjected to a temperature within the range between about 1000° F.and about 1500° F., preferably between about 1200° F. and 1400° F., andto a pressure between about 100 psia and about 1500 psia, preferablybetween about 200 psia and about 800 psia. Under such conditions thealkali metal constituents interact with the carbon in the carbonaceoussolids to form a carbon-alkali metal catalyst, which will under properreaction conditions equilibrate the gas phase reactions occurring duringgasification to produce additional methane and at the same time supplysubstantial amounts of additional exothermic heat in situ. Due to thegas phase equilibrium conditions existing as a result of thecarbon-alkali metal catalyst and due to the presence of equilibriumquantities of hydrogen and carbon monoxide injected with the steam nearthe lower end of the bed, the net reaction products will normallyconsist essentially of methane and carbon dioxide. Competing reactionsthat in the absence of the catalyst and the hydrogen and carbon monoxidewould ordinarily tend to produce additional hydrogen and carbon monoxideare suppressed. At the same time, substantial quantities of exothermicheat are released as a result of the reaction of hydrogen with carbonoxides and the reaction of carbon monoxide with steam. This exothermicheat tends to balance the endothermic heat consumed by the reaction ofthe steam with carbon, thereby producing an overall thermoneutralreaction. So far as the heat of reaction is concerned, the gasifier istherefore largely in heat balance. The heat employed to preheat the feedcoal to the reaction temperature and compensate for heat losses from thegasifier is supplied for the most part by excess heat in the gasesintroduced into the gasifier through lines 35 and 36. In the absence ofthe exothermic heat provided by the catalyzed gas phase reactions, thesegases would have to be heated to substantially higher temperatures thanthose employed here.

The carbon-alkali metal catalyst utilized in the process of theinvention is prepared by heating an intimate mixture of carbon and analkali metal constituent to an elevated temperature, preferably above800° F. In the process shown in the drawing and described above, theintimate mixture is prepared by impregnating the carbonaceous feedmaterial with an alkali metal-containing solution and then subjectingthe impregnated solids to a temperature above 800° F. in the gasifieritself. It will be understood that the alkali metal catalyst utilized inthe process of this invention can be prepared without impregnation ontothe carbonaceous solids to be gasified, and without heating in thegasifier. The heating step, for example, may be carried out in a solidfeed preparation zone or in an external heater. The carbonaceous solidsused will, in most instances, be the ones which are to be gasified butin some variations of the process carbonaceous materials other than thefeed solids may be used. In some cases inert carriers having carbondeposited on their outer surface may be used. Suitable inert carriersinclude silica, alumina, silica-alumina, zeolites and the like. Thecatalyst particles, whether composed substantially of carbon and analkali metal constituent or made up of carbon and an alkali metalconstituent deposited on an inert carrier, may range from fine powdersto coarse lumps, particles between about 4 and about 100 mesh on theU.S. Sieve Series Scale generally being preferred.

Any of a variety of alkali metal constituents or mixtures thereof can beused in preparing the carbon-alkali metal catalyst. Suitableconstituents include the alkali metals themselves and alkali metalcompounds such as alkali metal carbonates, bicarbonates, formates,oxalates, hydroxides, sulfides and mixtures of these and other similarcompounds. All of these are not equally effective and hence a catalystprepared from certain alkali metal constituents can be expected to givesomewhat better results under certain conditions than do others. Ingeneral, cesium, potassium, sodium and lithium salts derived fromorganic or inorganic acids having ionization constants less than about1×10⁻³ and alkali metal hydroxides are preferred. The cesium compoundsare the most effective, followed by the potassium, sodium and lithiumcompounds, in that order. Because of their high activity, relatively lowcost compared to cesium compounds and ready availability, potassiumcompounds or sodium compounds are generally employed. Potassiumcarbonate and potassium hydroxide are especially effective.

In the embodiment of the invention shown in the drawing, alkali metalconstituents and the carbonaceous solids are combined to form anintimate mixture by dissolving water soluble alkali metal compounds inan aqueous carrier, impregnating the carbonaceous solids with theresulting aqueous solution by soaking or spraying the solution onto theparticles and thereafter drying the solids. It will be understood thatother methods of forming such an intimate mixture may be used. Forexample, in some cases the carbonaceous material can be impregnated bysuspending a finely divided alkali metal or alkali metal compound in ahydrocarbon solvent or other inert liquid carrier of suitably lowviscosity and high volatility and thereafter treating the solids withthe liquid containing the alkali metal constituent. In other cases, itmay be advantageous to pelletize a very finely divided alkali metal oralkali metal compound with carbon in an oil or similar binder and thenheat the pellets to an elevated temperature. Other catalyst preparationmethods, including simply mixing finely divided carbonaceous materialwith a powdered alkali metal salt and thereafter heating the mixture tothe desired temperature can, in some cases, also be used.

The mechanisms which take place as the result of combining thecarbonaceous solids and alkali metal constituents and then heating themto elevated temperatures are not fully understood. Apparently, thealkali metal reacts with the carbon to form carbon-alkali metalcompounds and complexes. Studies have shown that neither thecarbonaceous solids nor the alkali metal constituents alone are fullyeffective for establishing equilibrium condition for gas phase reactionsinvolving steam, hydrogen, carbon monoxide, carbon dioxide, and methaneand that catalytic activity is obtained only when a compound or complexof the carbon and alkali metal is present in the system. Bothconstituents of the catalyst are therefore necessary. Experience hasshown that these catalystss are resistant to degradation in the presenceof sulfur compounds, that they resist sintering at high temperatures andthat they bring gas phase reactions involving the gases normallyproduced during coal gasification into equilibrium. As a result of theseand other beneficial properties, these catalysts have pronouncedadvantages over other catalysts employed in the past.

Referring again to the drawing, the gas leaving the fluidized bed isgasifier 32 passes through the upper section of the gasifier, whichserves as a disengagement zone where the particles too heavy to beentrained by the gas leaving the vessel are returned to the bed. Ifdesired, this disengagement zone may include one or more cycloneseparators or the like for removing relatively large particles from thegas. The gas withdrawn from the upper part of the gasifier through line37 will normally contain an equilibrium mixture at gasificationtemperature and pressure of methane, carbon dioxide, hydrogen, carbonmonoxide and unreacted steam. Also present in this gas are hydrogensulfide, ammonia and other contaminants formed from the sulfur andnitrogen contained in the feed material, and entrained fines. This rawproduct gas is introduced into cyclone separaor or similar device 38 forremoval of the larger fines. The overhead gas then passes through line39 into a second separator 41 where smaller particles are removed. Thegas from which the solids have been separated is taken overhead fromseparator 41 through line 42 and the fines are discharged downwardthrough dip legs 40 and 43. These fines may be returned to the gasifieror passed to the alkali metal recovery portion of the process.

In the system shown in the drawing, a stream of high ash content charparticles is withdrawn through line 44 from gasifier 32 in order tocontrol the ash content of the system and permit the recovery andrecycle of alkali metal constituents of the catalyst. The solids in line44, which may be combined with fines recovered from the gasifieroverhead gas through dip legs 40 and 43 and line 45, are passed toalkali metal recovery unit 46. The recovery unit will normally comprisea multistage countercurrent leaching system in which the high ashcontent particles are countercurrently contacted with water introducedthrough line 47. An aqueous solution of alkali metal compounds iswithdrawn from the unit through line 48 and recycled through lines 49,18 and 17 to feed preparation zone 14. Ash residues from which solublealkali metal compounds have been leached are withdrawn from the recoveryunit through lines 50 and may be disposed of as landfill or furthertreated to recover additional alkali metal constituents.

The gas leaving separator 41 is passed through line 42 to gas-gas heatexchanger 51 where it is cooled by indirect heat exchange with a gaseousmixture containing methane carbon monoxide, carbon dioxide, hydrogen andsteam introduced through line 94. The cooled gas is then passed throughline 53 into waste heat boiler 54 where it is further cooled by indirectheat exchange with water introduced through line 55. Sufficient heat istransferred from the gas to the water to convert it into steam, which iswithdrawn through line 95. During this cooling step, unreacted steam inthe gas from exchanger 51 is condensed out and withdrawn as condensatethrough line 57. The cool gas exiting waste heat boiler 54 thriugh line58 is passed to water scrubber 59. Here the gas stream passes upwardthrough the scrubber where it comes in contact with water injected intothe top of the scrubber through line 60. The water removes residualparticulates and absorbs ammonia and a portion of the hydrogen sulfidein the gas stream, and is withdrawn from the bottom of the scrubberthrough line 61 and passed to downstream units for further processing.The water scrubbed gas stream is withdrawn from the scrubber throughline 62 and is now ready for treatment to remove bulk amounts ofhydrogen sulfide and carbon dioxide.

The gas stream is passed from water scrubber 59 through line 62 into thebottom of hydrogen sulfide absorber 63. Here the gas passes upwardthrough the contacting zone in the absorber where it comes into contactwith a downflowing stream of a solvent introduced into the upper part ofthe absorber through line 64. The solvent will normally be a compoundthat under certain operating conditions will selectively absorb hydrogensulfide in preference to carbon dioxide. Examples of such solventsinclude the dimethyl ether of polyethylene glycol, methanol and thelike. If desired, the absorber may be provided with spray nozzles,perforated plates, bubble cap plates, packing or other means forpromoting intimate contact between the gas and the solvent. As the gasrises through the contacting zone, substantially all of the hydrogensulfide and a portion of the carbon dioxide are absorbed by the solvent,which exits the scrubber through line 65. The spent solvent containingprimarily hydrogen sulfide and small concentrations of othercontaminants is passed through line 65 to a stripper, not shown in thedrawing, where it is contacted with steam or other stripping gas toremove the hydrogen sulfide and other absorbed contaminants and therebyregenerate the solvent. The regenerated solvent may then be reused byinjecting it back into the top of the absorber via line 64.

A gas containing essentially methane, hydrogen, carbon monoxide andcarbon dioxide is withdrawn overhead from hydrogen sulfide absorber 63through line 66 and passed into the bottom of carbon dioxide absorber68. Here, the mixture of gases passes upward through the contacting zonein the absorber where it comes in contact with a downflowing stream ofsolvent introduced into the top portion of the absorber through line 69.Normally, the solvent will be the same solvent that is used in hydrogensulfide absorber 63. Absorber 68 will be operated under conditions suchthat the solvent absorbs almost all of the carbon dioxide from themixture of gases passed upward through the contacting zone in theabsorber. If desired, however, a different solvent than the one used inhydrogen sulfide absorber 63 may be used in absorber 68. Spent solventcontaining primarily carbon dioxide and small amounts of othercontaminants is withdrawn from the absorber through line 70 and passedto a stripper, not shown in the drawing, where it is contacted withsteam or other stripping gas to remove the carbon dioxide and otherabsorbed contaminants and thereby regenerate the solvent. Theregenerated solvent may then be reused by injecting it back into the topof the absorber via line 69. A clean gas having an intermediate Btuheating value and containing essentially methane, hydrogen and carbonmonoxide is withdrawn overhead from the carbon dioxide absorber throughline 71.

The intermediate Btu gas withdrawn overhead from carbon dioxide absorber68 is introduced into heat transfer unit 72 where it passes in indirectheat exchange with liquid methane introduced through line 73. Themethane vaporizes within the heat transfer unit and is discharged as agas through line 74. The vaporizing methane chills the intermediate Btugas, which is composed primarily of methane, hydrogen and carbonmonoxide, to a low temperature approaching that required forliquefaction of the methane contained in the gas, after which thechilled gas is passed through line 75 into cryogenic distillation unit76. Here the gas is further cooled by conventional means until thetemperature reaches a value sufficiently low to liquefy the methaneunder the pressure conditions existing in the unit. Compressors andother auxiliaries associated with the cryogenic distillation unit arenot shown. The amount of pressure required for the liquefaction stepwill depend in part upon the pressure at which the gasifier is operatedand the pressure losses which are incurred in various portions of thesystem. A liquid stream containing greater than about 95 weight percentmethane is withdrawn from the cryogenic unit through line 77 and passedthrough line 73 into heat transfer unit 72 as described earlier. Amixture of hydrogen, carbon monoxide and methane is withdrawn overheadfrom the cryogenic distillation unit through line 78. The cryogenic unitis normally operated and designed in such a manner that the gas removedoverhead will contain between about 2 and about 15 mole percent methane.It will be understood that the cryogenic unit does not necessarily haveto be a distillation column and instead can be any type of cryogenicunit in which a majority of the carbon monoxide and hydrogen isseparated from the methane in the gas that is fed to the unit. Forexample, the unit could be a simple one-stage cryogenic flash in whichcase the effluent removed in line 77 will be methane-rich and willcontain a signiciant amount of dissolved carbon monoxide and hydrogen.

As pointed out above, the mixture of carbon monoxide, hydrogen andmethane removed overhead from cryogenic unit 76 through line 78 willnormally contain only between about 2 mole percent and about 15 molepercent methane. It has been found that this synthesis gas is a suitablefeed material for the synthesis of methanol and thus facilitates theintegration of catalytic gasification with a methanol synthesis process.

The synthesis gas in line 78 is mixed with carbon dioxide introducedinto line 78 through line 79. Normally, the carbon dioxide is obtainedfrom the regeneration of the spent solvent removed from carbon dioxideabsorber 68 through line 70. Sufficient carbon dioxide is normallyinjected into line 78 in order to adjust the carbon oxides to hydrogenratio and/or activate the methanol synthesis catalyst. The mixture ofsynthesis gas and carbon dioxide is then combined with a cool recyclestream of hydrogen, carbon monoxide, methane and carbon dioxideintroduced into line 78 via line 80, and the combined streams arecompressed and passed to methanol synthesis reactor 81. Here thecombined gases are passed downwardly through fixed beds of methanolsynthesis catalyst at a temperature between about 400° F. and about 700°F., preferably between about 425° F. and about 575° F., and at apressure between about 400 psig and about 2000 psig. Under theseconditions and in the presence of the catalyst, the carbon monoxide andhydrogen in the mixture of gases react to form methanol. The carbondioxide in the gases may react slowly with hydrogen to form methanol andwater or carbon monoxide and water.

The methanol synthesis reactor will normally be comprised of a pressurevessel or series of such vessels, each containing a charge of catalystarranged in a continuous bed or several independently supported beds.The amount of catalyst provided in each reactor will depend upon thetemperature and pressure employed during the synthesis, the compositionof the feed gas to the reactor, and the degree of conversion of the feedgas to methanol in each catalyst bed. The space velocity employed in thereactor or reactors will vary from about 5000 to about 50,000,preferably from about 7000 to about 25,000, volumes of dried gas atstandard conditions per volume of per catalyst.

The catalyst employed in the reactor or reactors will normally bepartially reduced oxides of copper, zinc and chromium; zinc oxide andchromium oxides; zinc oxide and copper; copper and aluminum oxide orcesium oxide; zinc oxide and ferric oxide; zinc oxide and cupric oxide;a copper zinc alloy; the oxides of zinc, magnesium, cadmium, chromium,vanadium and/or tungsten with oxides of copper, silver, iron and/orcobalt; and the like. Either individual catalysts or mixtures ofcatalysts may be used. The catalyst may be finely ground, pelleted,granular, extruded with a binding agent or in any other suitable form.

The gaseous effluent from methanol synthesis reactor 81, which consistsprimarily of methanol, unreacted carbon monoxide and hydrogen, and smallamounts of carbon dioxide, methane and water is withdrawn through line82 and passed to heat exchanger 83 where the gases are cooled tocondense the methanol and any water formed in the methanol synthesisreactor. The cooled effluent exiting heat exchanger 83 through line 84is passed to methanol accumulator 85 where the condensed liquids areallowed to separate from the cooled gases. Methanol is removed from theaccumulator through line 86 and passed downstream for purification. Thedegree of purity required for the methanol product will depend upon theend use contemplated for the methanol. If the methanol is to be useddirectly as fuel, very little, if any, purification may be required.

The cool gas exiting methanol accumulator 85 through line 87 willnormally contain between about 20 and about 80 mole percent hydrogen,between about 2 and about 20 mole percent carbon monoxide, between about20 and about 60 mole percent methane and a small amount of carbondioxide. A portion of this gas is normally recycled directly to methanolsynthesis reactor 81. In general, between about 75 volume percent andabout 97 volume percent of the gas in line 87 is compressed and passedthrough lines 80 and 88 for reintroduction into methanol synthesisreactor 81. Recycle of this cool gas serves to prevent the temperaturein the methanol synthesis reactor from increasing to an undesired valueand also allows for further conversion of the carbon monoxide andhydrogen comprising the gas into methanol as these constituents againpass through the reactor.

The portion of the gas in line 87 that is not directly recycled tomethanol synthesis reactor 81 through lines 80 and 88 is normallyreferred to as a purge gas in conventional methanol synthesis processes.This gas is passed through line 89 to compressor 90 where its pressureis increased to a value from about 25 psi to about 150 psi above theoperating pressure in gasifier 32. The pressurized gas withdrawn fromcompressor 90 through line 91 is passed through tubes 92 located in theconvection section of steam reforming furnace 93. Here, the highpressure gas picks up heat via indirect heat exchange with the hot fluegases generated in the furnace. The purge gas is removed from tubes 92through line 94 and mixed with steam, which is generated in waste heatboiler 54 and injected into line 94 via line 95. The mixture of purgegas and steam is then passed through line 94 into gas-gas heat exchanger51 where it is heated by indirect heat exchange with the gasifier rawproduct gas removed from separator 41. The heated mixture is removedfrom exchanger 51 and passed through line 96 into steam reformingfurnace 93.

The preheated mixture of steam and purge gas in line 96 is introducedinto the internal tubes 97 of the steam reforming furnace where themethane in the purge gas reacts with steam in the presence of aconventional steam reforming catalyst. The catalyst will normallyconsist of metallic constituents supported on an inert carrier. Themetallic constituents will normally be selected from the Group VI-B andthe iron group of the Periodic Table of Elements and may be chromium,molybdenum, tungsten, nickel, iron or cobalt, and may include smallamounts of potassium carbonate or a similar compound as a promoter.Suitable inert carriers include silica, alumina, silica-alumina,zeolites and the like.

The reforming furnace is operated under conditions such that the methanein the purge gas will react with steam in the tubes 97 to producehydrogen and carbon monoxide according to the following equation:

    H.sub.2 O+CH.sub.4 →3H.sub.2 +CO

The temperature in the reforming furnace will normally be maintainedbetween about 1200° F. and about 1800° F., preferably between about 100°F. and about 300° F. above the temperature in gasifier 32. The outletpressure will range between about 10 and about 30 psi above the pressurein the gasifier. The mole ratio of steam to methane introduced into thereforming furnace will range between about 2:1 and about 15:1,preferably between about 3:1 and about 7:1. If the amount of steam addedvia line 95 to the purge gas being fed to the steam reforming furnace inline 94 is not sufficient, additional steam may be injected into line 94via line 101. Although the reforming furnace may be fired by anyavailable fuel gas, it is normally preferred to use all or a portion ofthe methane-rich gas removed from heat transfer unit 72. The requiredamount of gas is withdrawn from the heat transfer unit through line 74and passed directly to the firebox in the steam reforming furnace vialine 99.

The gaseous effluent stream from the steam reforming furnace, which willnormally be a mixture consisting primarily of hydrogen, carbon monoxide,carbon dioxide and unreacted steam and methane is passed, preferablywithout substantial cooling through lines 98, 36 and 33 into gasifier32. This stream is the primary source of the hydrogen, carbon monoxideand steam required in the gasifier in addition to the carbon-alkalimetal catalyst to produe a thermoneutral reaction that results in theformation of essentially carbon dioxide and methane. It is, therefore,desirable that the reforming furnace effluent contain sufficient carbonmonoxide and hydrogen to supply the substantially equilibrium quantitiesof those gases required in the gasifier and sufficient unreacted steamto provide substantially all the steam required by the reactions takingplace in the gasifier.

As pointed out previously, substantial quantities of exothermic heat arereleased in the gasifier as a result of the reaction of hydrogen withcarbon oxides and the reaction of carbon monoxide with steam. Since thecarbon monoxide and hydrogen in the reforming furnace effluent streamcomprises a substantial portion of the heat input to the gasifier,sufficient methane should normally be present in the feed to thereforming furnace so that enough carbon monoxide and hydrogen isproduced by steam reforming the methane to supply enough additionalcarbon monoxide and hydrogen to supplement the amount of those gasesthat are originally present in the purge gas from the methanol synthesisportion of the process that is fed to the reforming furnace. If there isinsufficient methane in this purge gas, a portion of the methane-richgas exiting heat transfer unit 72 may be used to supplement the methanein the purge gas. The required amount of methane-rich gas is withdrawnfrom the heat transfer unit through line 74 and passed via line 100 intoline 89 where it is mixed with the purge gas before it passes throughcompressor 90.

For the purposes of thermal efficiency, it is preferable that the steamreforming step of the process be utilized in such a manner as to obviatethe need for a separate preheat step. This may be achieved by operatingthe reforming furnace so that the heat content of the effluent issufficient to preheat the gasifier feed material to gasificationtemperature and maintain all the reactants at such temperature bycompensating for heat losses during gasification. Normally, this may beaccomplished if the temperature of the effluent is between about 100° F.and about 300° F. higher than the operating temperature in the gasifier.For optimum thermal efficiency, it is important that the effluent fromthe steam reforming furnace be passed to the gasifier in such a manneras to avoid substantial cooling. It will be apparent from the abovediscussion that the effluent from reforming furnace 93 will supplysubstantially all the heat required in the gasifier 32. The effluentwill not only contain sufficient sensible heat to preheat thecarbonaceous feed material to reaction temperature and maintain all thereactants at such temperature by compensating for heat losses duringgasification, but it will also contain sufficient amounts of carbonmonoxide and hydrogen which react in the gasifier to produce enoughexothermic heat to substantially balance the endothermic heat consumedby the reaction of the steam with carbon.

The methane concentration in the methane-rich gas removed from heattransfer unit 72 through line 74 will depend upon the type of cryogenicseparation unit 76 that is utilized in the process. If a cryogenicdistillation column is used, the methane-rich gas will normally containgreater than about 95 weight percent methane along with carbon monoxideand hydrogen. If a less efficient cryogenic separation is employed, thegas will contain greater amounts of carbon monoxide and hydrogen andwill normally be of an intermediate Btu heating value. As pointed outearlier, this methane-rich gas may be used as fuel for the steamreforming furnace and/or as a supplemental source of methane for thefeed to the reforming furnace. Any of the methane-rich gas which is notutilized for one or both of these purposes is withdrawn from the processas a gaseous by-product which may subsequently be used as a fuel tosupply the heat requirements of industrial plants or for other purposes.In some cases it may be desirable that none of the methane-rich gas beused as feed to the steam reforming furnace.

In conventional methanol synthesis processes, the portion of themethanol synthesis reactor effluent that is not recycled directly to themethanol synthesis reactor is removed as a purge gas in order to preventcarbon dioxide and methane from accumulating rapidly in the reactor andchoking the methanol synthesis reactions. This purge gas is normallysold as an intermediate Btu gas. Economics, however, militate againstthe direct burning of the carbon monoxide and hydrogen, which comprise asignificant portion of the purge gas. The integration of catalyticgasification with methanol synthesis in accordance with the process ofthe invention allows this purge gas to be utilized in a more efficientmanner since the carbon monoxide and hydrogen in the purge gas iseventually fed to the gasifier to provide a large portion of theexothermic heat required in the gasifier to supply the gasificationreactions. Furthermore, since this purge gas contains a significantamount of carbon monoxide and hydrogen, the size of the steam reformingfurnace needed to convert the methane in the purge gas into additionalcarbon monoxide and hydrogen is normally smaller than in other catalyticgasification processes that utilize a steam reforming furnace. Thus, theprocess of the invention results in a more efficient process whichutilizes the various gas streams generated therein in an economicallyattractive manner.

In the embodiment of the invention described above and shown in thedrawing, a cryogenic separation unit is utilized to separate methanefrom carbon monoxide and hydrogen. It will be understood that theprocess of the invention is not limited to a cryogenic separation unitand includes any type of separation zone in which a mixture of carbonmonoxide, hydrogen and methane is divided into a stream rich in carbonmonoxide and hydrogen, and a methane-rich stream.

It will be apparent from the foregoing that the invention provides aprocess for producing methanol via the catalytic gasification of coaland similar carbonaceous feed materials. The process of the inventionhas advantages over conventional methanol synthesis processes in that itresults in the efficient use of the methanol purge gas to supply heat tothe gasification process.

We claim:
 1. A process for the production of methanol from acarbonaceous feed material which comprises:(a) gasifying saidcarbonaceous feed material with steam in a gasification zone at agasification temperature between about 1000° F. and about 1500° F. andat a gasification pressure in excess of about 100 psia in the presenceof a carbon-alkali metal catalyst and added hydrogen and carbonmonoxide; (b) withdrawing from said gasification zone an effluent gascontaining methane, carbon monoxide, steam, hydrogen, carbon dioxide andhydrogen sulfide; (c) treating said effluent gas for the removal ofsteam, particulates, hydrogen sulfide and carbon dioxide to produce atreated gas containing primarily carbon monoxide, hydrogen and methane;(d) passing said treated gas to a separation zone wherein said treatedgas is separated into a methane-rich gas stream and a gas streamcontaining primarily carbon monoxide and hydrogen; (e) passing said gasstream containing primarily carbon monoxide and hydrogen produced instep (d) to a methanol synthesis zone wherein at least a portion of saidcarbon monoxide is reacted with at least a portion of said hydrogen inthe presence of a methanol synthesis catalyst to form methanol; (f)recovering methanol product from the effluent stream exiting saidmethanol synthesis zone, thereby leaving a gas comprised of carbonmonoxide, hydrogen, methane and carbon dioxide; (g) passing at least aportion of said gas stream comprised of carbon monoxide, hydrogen,methane and carbon dioxide produced in step (f) to a steam reformingzone wherein at least a portion of said methane in said gas stream isreacted with steam to produce hydrogen and carbon monoxide; and (h)passing the effluent from said steam reforming zone into saidgasification zone, thereby supplying said added hydrogen and carbonmonoxide required in said gasification zone.
 2. A process as defined byclaim 1 wherein said carbonaceous feed material comprises coal.
 3. Aprocess as defined by claim 2 wherein said carbon-alkali metal catalystis prepared by treating said coal with an alkali metal compound orcompounds and thereafter heating the coal to said gasificationtemperature in said gasification zone.
 4. A process as defined by claim1 wherein at least a portion of the gas comprising carbon monoxide,hydrogen, methane and carbon dioxide produced in step (f) is recycleddirectly to said methanol synthesis zone.
 5. A process as defined byclaim 1 wherein said gas stream containing primarily carbon monoxide andhydrogen produced in step (d) is mixed with carbon dioxide removed fromthe effluent gas from said gasification zone in step (c) prior topassing said gas stream to said methanol synthesis zone.
 6. A process asdefined by claim 1 wherein at least a portion of said methane-rich gasproduced in step (d) is mixed with said portion of the gas streamcomprised of carbon monoxide, hydrogen, methane and carbon dioxidereferred to in step (g) prior to passing said portion of said gas streamto said steam reforming zone.
 7. A process as defined by claim 1 whereinat least a portion of said methane-rich gas produced in step (d) is usedas fuel for said steam reforming zone.
 8. A process as defined by claim1 wherein between about 75 and about 97 volume percent of the gascomprising carbon monoxide, hydrogen, methane and carbon dioxideproduced in step (f) is recycled directly to said methanol synthesiszone and the remaining portion of said gas is passed to said steamreforming zone.
 9. A process as defined by claim 1 wherein saidseparation zone in step (d) comprises a cryogenic separation zone.
 10. Aprocess as defined by claim 9 wherein said cryogenic separation zonecomprises a cryogenic distillation column.